This invention relates to an open architecture superconducting magnet assembly for a magnetic resonance imager (hereinafter called "MRI"), and more particularly to a strengthened helium pressure vessel for use in MRI equipment.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold enables that the magnet coils to become superconducting, such that when a power source is initially connected to the coils to introduce a current flow though the coils, the current will continue to flow even after power is removed due to negligible coil resistance at superconducting temperatures, thereby maintaining a magnetic field. Superconducting magnets find wide application in the field of MRI.
Most MRI equipments utilize solenoidal magnets enclosed in a unitary cylindrical structure with a central bore opening for patient access. However, the patient is practically enclosed in the warm bore, which can induce claustrophobia in some patients. The desirability of utilizing an open architecture structure in which the patient is not essentially totally enclosed has long been recognized as a means of patient comfort and to enable interventional procedures by the surgeon or health care provider. Unfortunately, an open architecture structure of the type utilizing two spaced donut-type or cylindrical shaped annular coil assemblies to provide open space and a vertical or central patient access gap between the coil assemblies poses a number of technical problems and challenges. One problem and challenge is to provide a structure which will produce the very homogeneous magnetic imaging field required while adequately supporting the separated coil assemblies under the considerable electromagnetic forces and thermal forces encountered during operation. The electromagnetic forces include large axial magnetic forces between the subcoils within each donut coil assembly in addition to a very large axial net force between the two donuts. Such magnetic forces are balanced in the more conventional unitary symmetrical superconducting magnet assembly and act axially toward each other with the intervening solid connecting structure receiving and readily handling such forces which are basically in balance.
However, in the case of a separated pair of donut or annular magnet assemblies, the imbalanced axial magnetic forces of each donut acts directly on the helium pressure vessels including their end flanges resulting in high stresses and possible deformation of the vessels which degrades homogeneity of the resultant magnetic field and the quality of the imaging. This is particularly true in the case of desirable lightweight magnets including those used in portable MRI equipment where weight is a consideration leading to the desired use of lightweight metal pressure vessels.
The magnetic field in the imaging bore must be very homogeneous and temporally constant for accurate imaging. Even slight flexures of the helium vessels and/or their internal superconducting magnet coil wires alters the spatial distribution of current flow through the coils and homogeneity of the imaging magnetic field sufficiently to degrade the quality of images produced by the MRI imaging system.
There are a number of other problems, including problems of differential thermal expansion and contraction of materials, and of minimizing weight and cost. All of these overlapping and conflicting requirements must be satisfied for a practical and useful MRI superconducting magnet structure.